1. Field of the Invention
The present invention relates to an image processing device, an image processing method and a print system for correcting density unevenness in the sub-scanning direction, which is caused in a print device.
2. Description of Related Art
In a print device for printing an image on recording paper, density unevenness is caused due to various types of factors. In order to reduce the density unevenness, the following correction process is generally carried out. In the correction process, a test image is printed on recording paper, and the density unevenness is measured by optically reading the printed image. Then, the correction data for cancelling the measured density unevenness is prepared, and the image data to be printed is corrected in accordance with the prepared correction data.
The density unevenness is caused in the main scanning direction which is perpendicular to the conveying direction of the recording paper and in the sub-scanning direction which is the conveying direction of the recording paper. Therefore, in order to precisely correct the density unevenness, it is desired to correct the density unevenness not only in the main scanning direction but also in the sub-scanning direction.
For example, in Japanese Patent Application Publication No. 2009-192896, as a method for reducing the density unevenness both in the main scanning direction and in the sub-scanning direction, the following technology is disclosed. In the technology, test charts having each constant density in the main scanning direction and in the sub-scanning direction are printed on a plurality of pages so as to gradually change the density of the test chart printed in every page, and the density unevenness of each density is measured in the main scanning direction and in the sub-scanning direction to correct the density unevenness.
On the other hand, because there are various types of factors of the density unevenness, it is effective to correct the density unevenness which is caused by a main factor. In Japanese Patent Application Publication No. 2010-220182, the following technology is disclosed. In the technology, from an output image or an image processed in an image forming process, such as an image formed on a photoconductive drum or an intermediate transfer belt, the density unevenness is resolved into the factors thereof and the factors of the density unevenness are extracted. Then, by sequentially correcting each of the extracted factors in series, the contribution ratio of each factor to the entire density unevenness is estimated.
The density unevenness in the sub-scanning direction is caused by, for example, the non-uniformity of rotating parts of the print device, such as a photoconductive drum, a intermediate transfer belt, various types of rollers and the like, and the switching on/off of a fixing heater. Further, the density unevenness in the sub-scanning direction is often periodically caused.
Conventionally, in case that the density unevenness in the sub-scanning direction is corrected, for example, as shown in FIG. 15, a test image 202 which is printed on one sheet of recording paper 201 and which has the even density is optically read to obtain the measuring data. In accordance with the obtained measuring data, the profile 203 of the density unevenness in the sub-scanning direction is prepared. Then, by carrying out the frequency analysis or the like for the profile 203, the periodic density unevenness in the sub-scanning direction is extracted and corrected.
However, in the density unevenness in the sub-scanning direction, there is density unevenness having a long period which cannot be contained in one sheet of recording paper 201. Therefore, in the conventional method using the measuring data obtained from the test image 202 printed on one sheet of recording paper 201, the above long period density unevenness in the sub-scanning direction cannot be removed.
As the factor of the long period density unevenness in the sub-scanning direction, for example, parts of the print device, which have the lengths longer than that of one sheet of recording paper, such as an intermediate transfer belt, the switching-on timing of a fixing heater, the toner supply timing and the like are considered.
In the technology disclosed in Japanese Patent Application Publication No. 2009-192896, a plurality of sheets of recording paper are used. However, the density unevenness for one density is measured by using only one sheet of recording paper. Further, in Japanese Patent Application Publication No. 2010-220182, the measuring data is obtained, by using one sheet of recording paper. These technologies are not suitable for the long period density unevenness in the sub-scanning direction.